Processing of steel bars after hot rolling

ABSTRACT

Steel bars are rapidly cooled from hot rolling temperature to a temperature of about 1,500* F. and are then cooled at a much lower rate to a temperature below their lower transformation temperature to minimize their compressive flow strength and improve their cold-heading and forming properties.

United States Patent [191 Grozier et al.

[ Apr. 1,1975

PROCESSING OF STEEL BARS AFTER HOT ROLLING Inventors: John David Grozier, Bethel Park;

John Ray Bell, Pittsburgh, both of Pa.

Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh. Pa.

Filed: Feb. 23, 1971 Appl. No.: 118,087

Related U.S. Application Data Continuation-impart of Scr. No. 768,163. Oct. 16, 1968. abandoned.

U.S. Cl. 148/12 C, 148/156 Int. Cl C2ld 7/14 Field of Search 148/12, 156

References Cited UNITED STATES PATENTS 12/1961 Kopec et a1 148/156 McLean et a1 148/12 Easter et a1... 148/12 McLean 148/156 Dopper et 148/156 Hill 148/156 Primary ExaminerW. Stallard Attorney. Agent, or FirmT. A. Zalenski ABSTRACT 3 Claims, No Drawings PROCESSING OF STEEL BARS AFTER HOT ROLLING This application is a continuation-in-part of our ap- This equation shows that a decrease in the gran size and decrease in the pearlite and silicon contents of a carbon steel will improve its cold-forming behavior. There are two methods to accomplish this in practice: l to pli i n Se iled O 6, 96 and 5 change the chemistry of the steel. or (2) to alter the now abandoned. structure of the steel through processing changes. If it This invention rel t t th manufacture f h is not possible to lower the carbon, manganese, or silirolled steel bars. It is more particularly concerned with C011 Contents of 11 g r example. here the Conmethods of processing such bars imm di t l ft h t tents of these elements are determined by hardenability rolling to improve their c ld-headin d f i g requirements, improvements in the cold-forming berti havior of hot-rolled steels can only be accomplished ei- The steel bars with which our invention is concerned ther l l pearhte content or by deFreaSmg are ordinary low to medium carbon steel bars intended femte gram 5128 through Control of Processmg for cold heading, forging or upsetting. The greater pro- 15 g? h H, b h d portion of these is made from steels of about 0.20 perurmg m m on h F s 4 9 L L cent to 0.45 percent carbon content. These steels are Hons per p'lss are i he austenmc r S128 qfter generally straight carbon steels, but may contain small i f s i l f i dec'riases a a amounts of alloys which affect hardenability. In small {gs 2552 3: T s fip f i e oun diameters such bars are generally coiled hot, but in 30 8 ys n a es p ace so p idly that It Is not possible to quench deformed austemte larger sizes are processed as straight lengths or strand. to .1 lower temperature and allow it to recrystallize to Historically: h -Ew Properties f E bars a smaller grain size. For the same reason we have found have OPUmIZed y glvmg h 5Phem1d1mg that the hot deformed structure of austenite in plain T hl$ treatment f ll g and carbon steels cannot be utilized to refine the ferritic expensive. It Is the principal ob ect of our invention to grain structure by direct transformation to ferrite from avoid this treatment while obtaining the properties recrystallized austenite. which it imparts. It is another object to provide a pro- Table I summarizes the temperature dependence in cess for treating bars directly from the hot rolling mill 1030 steel of austenitic grain size (ASTM No.) immediwhich optimizes their cold-heading properties. It is anately after complete recrystallization at the times other object to provide such a process adapted for bars noted, and after one hour, at the hot rolling temperawhich are coiled hot. It is another object to provide tures noted:

TABLE l TIME (SEC) FOR AUSTENITIC GRAIN SIZE (ASTM NO.) FINISHING COMPLETE AFIER AFTER TEMPERA- RECRYSTALLl- RECRYSTALLlZATlON ONE HOUR TURE ZATION 2000" F. 0.2 2.3 0.5 I900 F. 0.5 3.5 1.0 1800 F. 0.4 4.5 2.5 I700" F. 0.6 5.4 4.0 1600" F. 1.2 6.3 5.4 1500 F. 2.0 7.0 6.5

such a process for bars which are processed as straight It is seen that considerable grain growth occurs at lengths or strand. high temperatures if a steel is held at these tempera- The two properties of a steel that characterize its betures after roumg} however y g gram growth 9?- havior during cold forming are its compressive flow IfIthe Steel Cooled to The austbelmtlc stress (a measure of its strength) and its fracture strain g f h gz' g i 1 d F?" compalra g to at failure (a measure of its ductility). Low and medium t Ose 9? 0 i a E g i g carbon steels, free of excessive inclusions and surface 3253:; i g s s were re ea 6 0 ese defects, can be upset at least 70 percent without evidence of external cracking. Since cold-forming equip- We 9 found that deslrable colq'headmgfpwpep ent is in eneral not desi ned to exceed these strains Hes are lmparted to .Steel bars by coolmg them rom hot m g g rolling temperature In two steps or stages, the first comduring each cold-forming operation, the strain capacity prising rapid cooling to a temperature of about 1 0 of these steels is usually not a problem. The compres- F and the second comprising slow cooling from that sIve strength, however, Is mportant since this reflects temperature through the transformation range of the how large a load the steel Imparts to the die and punch Steel durmg the cold'formmg operauon' A hot bar cools by heat loss from its surface. While We have found that the compr lve flO Stress at this is going on the temperature of the center of the bar large strains depends on the following factors In accori al hi he th the t r t re f th bar s rdance with the following relationship: face. In a large bar being rapidly cooled this tempera- Flow Stress =95 .3 (P Pe8flture differential is considerable. To insure minimum (p r n (1000 P Percent P- grain growth it is desirable to cool the bar coming from set) where d in the grain size term (d is the mean linear intercept of ferrite in inches, and percent Si is the silicon content of the steel.

the mill at l,800 F. to 2,000 F. to a temperature which is above its lower transformation temperature as rapidly as possible. Too rapid a quench, however, will transform the surface of the bar into bainite or even martensite, both of which structures are undesirable in cold-heading steels, and we adjust the cooling of the bar to avoid this condition. There is no difficulty in this respect if the surface of the bar is not allowed to fall below about 1,200 F. We have found that a inch diameter bar allowed to cool in still air at room temperature from a mill finishing temperature in the range above mentioned to a temperature of about l,500 F. exhibits tolerable grain growth. We adjust the rapid cooling of our bars to a rate not less than that of the /8 inch diameter bar above mentioned. The measured rate of cooling at the center of such a bar is about 300 F. per minute. Our rapid cooling step is notlimited to air cooling, either in still air or by forced draft. We also utilize water cooling, especially for bars of larger diame' ter. but adjust it to bring about cooling at the rates above set out.

The rapid cooling step of our process is carried out with the bar in strand form or in sheared lengths as it comes from the rolling mill. Bar rolling mills are conventionally provided with run-out tables or with cooling beds on which the hot bars are delivered. We provide these run-out or cooling beds with means for directing cooling water or air against the hot bars to cool them in the way we have described.

Bars that are coiled continue to cool in the coil but because of the mass of the coil such cooling continues slowly. We have found that the rate of that cooling and the temperature from which the cooling starts influence the structure and the properties of the bar quite significantly. Table II tabulates data illustrating the effect of coiling temperature and cooling rate on the structure. hardness. and compressive flow stress of a bar of 10B22 steel. This steel analyzed 0.24 percent carbon, 0.82 percent manganese, 0.19 percent silicon, 0.021 percent aluminum, 0.001 percent boron and the remainder iron.

scribed for coils, or alternatively, the bar either as strand or as successive sheared lengths is passed continuously through a gradient furnace or cooling chamber. This furnace or chamber is provided with heating means and/or heat insulation which are adjusted so that the bars enter at temperatures slightly below those they attain at the end of the fast cooling stage and as they move through cool at the rate and to the exit temperature which we describe.

The first cooling step of our process is therefore adapted to prevent austenite grain growth after hot rolling and the second cooling step is adapted to cause the austenite to transform to ferrite plus pearlite slowly enough to result in minimum pearlite content of the resulting microstructure. Those skilled in the metallurgical art know that for a given steel composition there is a theoretical minimum proportion of pearlite which can be obtained, the equilibrium pearlite content, which can be calculated from the iron-carbon phase diagram. The conditions of our process are selected so as to provide a pearlite content as near to this minimum as is commercially feasible for steels of cold-heading composition range. The embodiment of our process presently preferred by us comprises rapidly cooling the bar from conventional hot-rolling finishing temperatures, around 1,800 to l,900 F., to a temperature not greater than about l,500 F. and not lower than the transformation temperature of the steel, followed by slow cooling through the lower transformation temperature of the steel to a temperature of about 1,200 F. or lower.

The microstructure and properties listed in Table II of the bars coiled at l,500 F. and cooled from that temperature at a rate of 50 F. per hour are very desirable in cold-heading steel. Bars coiled at l,500 F. and cooled from that temperature at a rate of 500 F. per hour are only marginally worse, but those coiled at TABLE II COILING** "/1 HARD- TE IIYIUPEIIEQA- COOLING PEARL- GRAIN SIZE NESS COMPRESSIVE FLOW STRESS 1000 PSI); e =TRUE STRAIN (F.) RATE ITE D"( IN."') R F05 e=0.2 e=0.4 E=0.6 e=0.8 =l .0

I900 50F/HR. 18 70 61 84 92 96 103 115 1700 F/HR. 33 30 63 93 97 104 116 I500 50F/HR. 26 31 70 61 83 92 96 I02 117 I900 500F/HR. 31 30 73 66 87 96 I00 105 I I6 1700 500F/HR. 30 31 74 66 87 I00 I I8 1500 500F/HR. 27 32 72 64 86 97 I00 107 121 1900 80F/MIN. 32 35 81 '71 93 I01 106 112 126 1700 80F/MIN. 28 38 81 72 94 I02 106 113 128 I500 80F/MIN. 30 38 81 72 94 I03 108 I I6 I34 AIR COOLED 6F/SEC. 43 49 90 86 107 I 16 127 I42 The finishing temperature of all bars was l900I'-. For I700F. and I500F. coiling temperatures. bars were air cooled from 1900F. to these temperatures at 6F/SE(.

From Table II it may be seen that bars coiled at 55 l,500

l,500 F., the lowest temperature shown, and cooled in the coil at a rate of'50 F. per hour, the slowest rate shown, had the smallest grain size, the smallest amount of pearlite in their microstructure, the lowest hardness, and the lowest compressive flow stress at various values of strain.

The slow cooling step of our process is carried out in different ways, depending on the form in which the bar is delivered. As we have mentioned, small diameter bars are conventionally coiled. Such coils range from less than 1,000 to as much as 3,000 pounds in weight.

Bars in the'form of strand or of sheared lengths are slow cooled by our process in the same way as is de- F. and cooled at a rate of 80 F. per minute, or 4,800 F. per hour, are quite noticeably inferior. We prefer, therefore, to cool bars coiled at a temperature of l,500 or thereabouts at a rate not greater than about 10 F. per minute. Large coils on the order of 3,000 pounds or greater will cool at this rate or slower in still air at room temperature because of their mass. Smaller coils cool at higher rates. For example, we have measured the cooling rate of a 1,450 pound coil of 17/32 inch diameter bar in the temperature range of l,500 F. to 1,200 F. as 930 F. per hour. We retard the rate of cooling of small coils by placing them inside insulated boxes or covers until they have cooled to a safe temperature.

Cooling should proceed at the desired rate above mentioned until all the austenite in the steel is transformed into ferrite and pearlite. Those skilled in the metallurgical art known that this transformation takes place over a range of temperatures that varies with the carbon content of the steel and the speed of the cooling. For low and medium carbon steels with which this invention is concerned. and cooling rates not greater than about F. per minute. a lower temperature for the slow cooling range of about 1200 F. is satisfactory. Below that temperature the steel may be cooled at any rate desired without affecting its cold heading properties.

We claim:

I. The method of processing steel bars of coldheading grade immediately after hot rolling to produce optimum cold-heading properties. the hot-rolling finishing temperature being above l,500F., including a fast cooling step followed by a slow cooling step. the

fast cooling step comprising cooling the hot bar to a LII temperature not greater than about l,500 F. but above its lower transformation temperature at a cooling rate not less than about 300 F. per minute but not great enough to form bainite or martensite at the bar surface. and the slow cooling step comprising cooling the bar through its lower transformation temperature to a temperature of not more than about l,200 F. at a cooling rate not greater than about 10 F. per minute.

2. The method of claim 1 in which the hot bar is coiled at the conclusion of the fast cooling step and the slow cooling step is carried out with the bar in coil form.

3. The method of claim 1 in which the slow cooling step is accomplished by passing the bar in strand form through a zone of temperature decreasing from a value above the lower transformation temperature at its entry end to a value below the lower transformation temperature at its exit end. 

1. THE METHOD OF PROCESSING STEEL BARS OF COLD-HEADING GRADE IMMEDIATELY AFTER HOT ROLLING TO PRODUCE OPTIMUM COLDHEADING PROPERTIES, THE HOT-ROLLING FINISHING TEMPERATURE BEING ABOUT 1,500*F, INCLUDING A FAST COOLING STEP FOLLOWED BY A SLOW COOLING STEP, THE FAST COOLING STEP COMPRISING COOLING THE HOT BAR TO A TEMPERATURE NOT GREATER THAN ABOUT 1,500*F, BUT ABOVE ITS LOWER TRANSFORMATION TEMPERATURE AT A COOLING RATE NOT LESS THAN ABOUT 300*F, PER MINUTE BUT NOT GREAT ENOUGH TO FORM BAINITE OR MARTENSITE AT THE BAR SURFACE, AND THE SLOW COOLING STEP COMPRISING COOLING THE BAR THROUGH ITS LOWER TRANSFORMATION TEMPERATURE TO A TEMPERATURE OF NOT MORE THAN ABOUT 1,200*F. AT A COOLING RATE NOT GREATER THAN ABOUT 10*F. PER MINUTE.
 2. The method of claim 1 in which the hot bar is coiled at the conclusion of the fast cooling step and the slow cooling step is carried out with the bar in coil form.
 3. The method of claim 1 in which the slow cooling step is accomplished by passing the bar in strand form through a zone of temperature decreasing from a value above the lower transformation temperature at its entry end to a value below the lower transforMation temperature at its exit end. 